Email script - ${lead.Interest} contains "A"

<ul>

#if(${{lead.Interest__c}}.indexOf('Performance'))

    <li>Performance</li>

#end

Should post to Products, not Champions (this space is to discuss the Champions program itself).

For a case-sensitive partial match you want

#if( $lead.Interest__c.contains("Performance") )

Note this will also match on "Power, LowPerformancer, Strength".

Matching that is less prone to error uses a regex:

#set( $item = "Performance" )
#if( $lead.Interest__c.matches("(?i)(.*)(^|,)\s*${item}\s*(,|$)(.*)") )

In general, "list-like" strings should be avoided unless you can make absolutely sure that the delimiter ("," in this case) cannot appear unescaped within an item. That is, if an item has a comma inside it -- which is totally possible with a string unless you control all the input -- parsing becomes problematic. Instead use a known structure like a JSON array.

(Geoff, you don't need curlies around a token if it isn't enclosed within quotes or output.)

ETA: Putting the matching item in a separate var will be more maintainable.